Investment mold having metallic donor element

ABSTRACT

An investment mold includes a refractory investment wall that has a surface region that bounds at least a portion of a mold cavity for receiving a molten metallic material. At least a portion of the surface region includes at least one metallic donor element with respect to the molten metallic material. Upon exposure of the surface region to the molten metallic material, the metallic donor element dopes into the molten metallic material. The method can be used to fabricate a cast article. The cast article can include a body formed of a metallic material having at least one reactive metallic element with respect to reactive-element-loss in an investment casting process. The body can include at least one internal passage that has a surface region that is compositionally rich in the reactive metallic element, which is doped into the body from the surface region of the refractory investment wall.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application No. 61/921,097, filed Dec. 27, 2013.

BACKGROUND

This disclosure relates to investment casting. Investment casting is known and used to cast metallic components with relatively complex geometries. For example, gas turbine engine components, such as airfoils, are fabricated by investment casting. For cast components that have internal passages, the internal passages can be formed using a core that represents a positive projection of negative features that are to be formed in the casting process. A wax pattern is provided around the core in the geometry of the component to be cast. A refractory shell is formed around the wax pattern and the wax is then removed to form a mold cavity between the core and the shell. Molten metal is poured into the cavity. After solidification of the metal, the shell and core are removed using known techniques to release the cast component.

SUMMARY

An investment mold according to an example of the present disclosure includes a refractory investment wall having a surface region bounding at least a portion of a mold cavity for receiving a molten metallic material. At least a portion of the surface region includes at least one metallic donor element with respect to the molten metallic material, such that upon exposure of the surface region to the molten metallic material the at least one metallic donor element dopes into the molten metallic material.

In a further embodiment of any of the foregoing embodiments, the at least one metallic donor element is selected from the group consisting of yttrium, lanthanum, hafnium, erbium, calcium, magnesium, and combinations thereof.

In a further embodiment of any of the foregoing embodiments, the at least one metallic donor element includes yttrium.

In a further embodiment of any of the foregoing embodiments, the refractory investment wall is an investment core having a refractory body and a coating of the at least one metallic donor element thereon.

In a further embodiment of any of the foregoing embodiments, the surface region includes an amount of the at least one metallic donor element sufficient to increase a sub-targeted amount of at least one metallic element in the molten metallic material at least to a targeted amount.

In a further embodiment of any of the foregoing embodiments, the at least one metallic donor element includes multiple metallic donor elements.

A method of controlling composition of a cast component in an investment casting process according to an example of the present disclosure includes introducing a molten metallic material into a mold cavity. The molten metallic material initially has a sub-targeted amount of at least one metallic element. Doping at least one metallic donor element from a surface region of a refractory investment wall bounds at least a portion of the mold cavity into the molten metallic material to increase the sub-targeted amount at least to a targeted amount of the at least one metallic element.

In a further embodiment of any of the foregoing embodiments, the at least one metallic donor element is selected from the group consisting of yttrium, lanthanum, hafnium, erbium, calcium, magnesium, and combinations thereof.

In a further embodiment of any of the foregoing embodiments, the at least one metallic donor element includes yttrium.

In a further embodiment of any of the foregoing embodiments, the refractory investment wall is an investment core within the mold cavity, the refractory investment wall having a refractory body and a coating of the at least one metallic donor element on the refractory body.

In a further embodiment of any of the foregoing embodiments, the at least one metallic donor element includes multiple metallic donor elements.

An investment cast article according to an example of the present disclosure includes a body formed of a metallic material having at least one reactive metallic element with respect to reactive-element-loss in an investment casting process. The body includes at least one internal passage having a surface region that is compositionally rich in the at least one reactive metallic element.

In a further embodiment of any of the foregoing embodiments, the body has a single crystal microstructure or a directionally solidified microstructure.

In a further embodiment of any of the foregoing embodiments, the at least one reactive metallic element is selected from the group consisting of yttrium, lanthanum, hafnium, erbium, calcium, magnesium, and combinations thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the present disclosure will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows.

FIG. 1 illustrates an example investment mold.

FIG. 2 illustrates a representative portion of a refractory investment wall of the investment mold of FIG. 1.

FIG. 3 illustrates an example investment cast article.

FIG. 4 illustrates a method of controlling composition of a cast component in an investment casting process.

DETAILED DESCRIPTION

Articles can be cast in investment molds from a molten metallic alloy. One example class of alloys useful for gas turbine engine articles are superalloys. Superalloys are nickel- or cobalt-based alloys. When the alloy is in a molten state, alloy elements can react with the materials of the walls of a pourcup (used to pour the molten alloy into an investment mold), the walls of the investment mold, or both. The reaction results in the loss of the element or elements from the composition of the alloy. Thus, even though an initial composition may meet composition requirements, the loss of one or more of elements from side reactions can reduce the alloy composition below required levels. A mold operator can add an additional amount of the element into the molten alloy to mitigate the loss, which can increase the complexity of the process and add cost. As will be described, the examples herein provide an investment mold having at least one metallic donor element to dope a molten alloy and limit element loss, which can reduce process complexity and costs.

FIG. 1 schematically illustrates selected portions of an example investment mold 20. In this example, the investment mold 20 is configured for casting a gas turbine engine article, such as an airfoil. For example, the article can be cast with a single crystal or directionally solidified microstructure. It is to be understood, however, that the investment mold 20 is not limited to airfoils or gas turbine engine articles, and the examples herein will also benefit other kinds of investment cast articles.

In the illustrated example, the investment mold 20 includes a mold cavity 22 that is generally surrounded by a refractory shell 24 (hereafter “shell 24”). A refractory core 26 (hereafter “core 26”) is situated within the mold cavity 22 and serves to form internal passages in the cast component. The shell 24 and the core 26 include refractory investment walls 28 that bound and define the mold cavity 22. As can be appreciated, some components may not have internal passages and may therefore not utilize the core 26. For example, the term “refractory” refers to a material that retains good strength at high temperatures (see also ASTM Volume 15.01 Refractories; Activated Carbon, Advanced Ceramics), such as above a temperature of 1,000° F. (811 K; 538° C.). In a further example, the refractory investment walls 28 are walls that, in the cast-ready state include, by weight, a total composition having a predominant amount of refractory material or materials, and in some examples 75% or greater, or 90% or greater, by weight of refractory material or materials. As can be further appreciated, the refractory investment walls 28 can be uni- or multi-layered.

FIG. 2 illustrates a representative portion of one of the refractory investment walls 28 of the investment mold 20. The refractory investment wall 28 includes a surface region 30 that bounds at least a portion of the mold cavity 22 for receiving a molten metallic material, represented at M in the figures. The surface region 30 includes at least one metallic donor element 32 with respect to the molten metallic material such that upon exposure of the surface region 30 to the molten metallic material, the metallic donor element or elements 32 dope into the molten metallic material.

The metallic donor element or elements 32 are selected according in correspondence with the reactive elements of the selected alloy being cast in a given investment mold system. As can be appreciated, the particular elements and the reactivity of such elements of a given alloy may vary between different investment mold systems, depending upon the alloy composition and investment mold composition, for example. The reactive elements can be identified experimentally through metallurgical compositional analysis. Once identified, the identified reactive metal or metals can be provided as the metallic donor element or elements 32 of the refractory investment wall 28. Thus, by providing the surface region 30 with the reactive elements as the metallic donor element or elements 32, these elements can be doped into the molten metallic material and can mitigate the loss of the reactive elements.

In a further example, the metallic donor element or elements 32 of the refractory investment wall 28 can be used as the sole source of the element in the alloy. For example, the alloy, prior to investment casting, does not include any of the element, but the element is doped into the alloy during investment casting to provide a targeted amount of the element in the alloy composition.

The metallic donor elements can be selected from yttrium, lanthanum, hafnium, erbium, calcium, magnesium, and combinations thereof. Thus, one or more of these elements can be included in the surface region 30 of the refractory investment wall 28 to dope into the molten metallic material during an investment casting process.

In a further example, the surface region 30 can include an amount of the one or more metallic donor elements 32 that is sufficient to increase a sub-targeted amount of the one or more metallic elements in the molten metallic material at least to a predetermined, targeted amount. For example, the amount of the metallic donor element 32 that dopes into the molten metallic material during a given casting process can be determined experimentally. Based on the amount that dopes into the molten metallic material, a corresponding amount can be provided in the surface region 30 of the refractory investment wall 28. In one example, the surface region is pure or substantially pure metallic donor element or elements 32, and the thickness of the surface region is controlled to provide a desired amount of the metallic donor element or elements 32 to the molten metallic material. Thus, the selected composition of the surface region 30 with regard to the metallic donor elements can correspond to the targeted composition of such elements in the molten metallic material.

As indicated above, the refractory investment wall 28 can be a wall of the shell 24, the core 26, or both. If the refractory investment wall 28 is a wall of the core 26, the core 26 will form an internal passage in the cast article. FIG. 3 shows a representative portion of a cast article 40 that includes a body 42 formed of a metallic material and an internal passage 44 that is formed using the core 26 with the surface region 30 as described herein. For example, the body 42 of the article 40 has a single crystal or directionally solidified microstructure. The metallic material of the body 42 has a sub-targeted amount of a metallic element. When the article 40 is cast, the portion of the core 26 that includes the refractory investment wall 28 and the surface region 30 dopes the metallic donor element or elements 32 into the body 42. Because the core 26 forms the internal passage 44, the surface region 30 is immediately adjacent the walls of the passage 44. Thus, the metallic donor element or elements 32 dope into the surface region 46 around the passage 44. The surface region 46 becomes compositionally rich in the metallic element from the surface region 30 of the refractory investment wall 28.

FIG. 4 illustrates an example method 50 of controlling composition of a cast component in an investment casting process, aspects of which have also been described above. The method 50 includes an introduction step 52 and a doping step 54. In the introduction step 52, the molten metallic material, M, is introduced into the mold cavity 22. The molten metallic material initially has a sub-targeted amount of at least one metallic element. In the doping step 54, at least one metallic donor element is doped from the surface region 32 of the refractory investment wall 28 bounding at least a portion of the mold cavity 22 into the molten metallic material. The doping of the metallic donor element increases the sub-targeted amount at least to a predetermined, targeted amount.

Although a combination of features is shown in the illustrated examples, not all of them need to be combined to realize the benefits of various embodiments of this disclosure. In other words, a system designed according to an embodiment of this disclosure will not necessarily include all of the features shown in any one of the Figures or all of the portions schematically shown in the Figures. Moreover, selected features of one example embodiment may be combined with selected features of other example embodiments.

The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. The scope of legal protection given to this disclosure can only be determined by studying the following claims. 

What is claimed is:
 1. An investment mold comprising: a refractory investment wall having a surface region bounding at least a portion of a mold cavity for receiving a molten metallic material, at least a portion of the surface region including at least one metallic donor element with respect to the molten metallic material, such that upon exposure of the surface region to the molten metallic material the at least one metallic donor element dopes into the molten metallic material.
 2. The investment mold as recited in claim 1, wherein the at least one metallic donor element is selected from the group consisting of yttrium, lanthanum, hafnium, erbium, calcium, magnesium, and combinations thereof.
 3. The investment mold as recited in claim 1, wherein the at least one metallic donor element includes yttrium.
 4. The investment mold as recited in claim 1, wherein the refractory investment wall is an investment core having a refractory body and a coating of the at least one metallic donor element thereon.
 5. The investment mold as recited in claim 1, wherein the surface region includes an amount of the at least one metallic donor element sufficient to increase a sub-targeted amount of at least one metallic element in the molten metallic material at least to a targeted amount.
 6. The investment mold as recited in claim 1, wherein the at least one metallic donor element includes multiple metallic donor elements.
 7. A method of controlling composition of a cast component in an investment casting process, the method comprising: introducing a molten metallic material into a mold cavity, the molten metallic material initially having a sub-targeted amount of at least one metallic element; and doping at least one metallic donor element from a surface region of a refractory investment wall bounding at least a portion of the mold cavity into the molten metallic material to increase the sub-targeted amount at least to a targeted amount of the at least one metallic element.
 8. The method as recited in claim 7, wherein the at least one metallic donor element is selected from the group consisting of yttrium, lanthanum, hafnium, erbium, calcium, magnesium, and combinations thereof.
 9. The method as recited in claim 7, wherein the at least one metallic donor element includes yttrium.
 10. The method as recited in claim 7, wherein the refractory investment wall is an investment core within the mold cavity, the refractory investment wall having a refractory body and a coating of the at least one metallic donor element on the refractory body.
 11. The method as recited in claim 7, wherein the at least one metallic donor element includes multiple metallic donor elements.
 12. An investment cast article comprising: a body formed of a metallic material having at least one reactive metallic element with respect to reactive-element-loss in an investment casting process, the body including at least one internal passage having a surface region that is compositionally rich in the at least one reactive metallic element.
 13. The investment cast article as recited in claim 12, wherein the body has a single crystal microstructure or a directionally solidified microstructure.
 14. The investment cast article as recited in claim 12, wherein the at least one reactive metallic element is selected from the group consisting of yttrium, lanthanum, hafnium, erbium, calcium, magnesium, and combinations thereof. 